A fuel cell is a power generation system for producing electrical energy through an electrochemical reaction of a hydrogen- or hydrocarbon-based fuel and an oxidizer, typified by oxygen. Since energy is directly obtained through an electrochemical reaction rather than through fuel combustion, high power generation efficiency and less pollution result, and thus, thorough research into the practical application of fuel cells is ongoing. Furthermore, a fuel cell is characterized in that a chemical reactant may be received from the outside to thus continuously generate power even without an additional charging process. The kinds of fuel cells are classified into, depending on the type of electrolyte, solid oxide fuel cells, phosphoric acid fuel cells, polymer electrolyte fuel cells, direct methanol fuel cells, and the like.
A fuel cell is typically configured to include a stack structure in which an electrolyte-electrode composite layer comprising electrodes, a catalyst layer, and a thin-film layer and a bipolar plate are alternately stacked. Such a stack structure includes two bipolar plates having flow channels so as to allow a reactive gas to flow therethrough. The bipolar plates function to supply a hydrogen fuel and oxygen to the electrolyte-electrode composite layer, collect current, and prevent the risk of explosion and combustion from occurring due to direct contact of hydrogen and oxygen, and thus are required to possess low gas permeability and high electrical conductivity. In particular, the bipolar plates are required to exhibit high electrical conductivity, phosphoric acid resistance for withstanding the strong corrosiveness of phosphoric acid, and high thermal conductivity, to thus enable the production of energy using waste heat and exhibit high strength.
Moreover, a fuel cell has the highest efficiency by means of cogeneration technology for producing heat and electricity, and is characterized in that a hydrocarbon-based fuel is reformed into hydrogen and then used, and thus the amount of harmful material from exhaust gas is very low compared to general thermal power generation. A phosphoric acid fuel cell operates at a relatively high temperature under phosphoric acid conditions, and a bipolar plate suitable therefor is required to exhibit high heat resistance and durability and low electrical resistivity.
Such a fuel cell is configured to include a gas-impermeable layer comprising a graphite conductor having a particle size of 0.01˜50 μm and a binder and a gas-permeable layer having a flow-channel pattern formed on one or both sides of the gas-impermeable layer and comprising a graphite conductor having a particle size of 100˜300 μm and a binder, as disclosed in Korean Patent No. 10-0805989, entitled “Bipolar plate for fuel cell and Stack for fuel cell comprising the same”. However, as in the conventional technique, in the case where a bipolar plate is manufactured by compressing only graphite having a large size, phosphoric acid resistance is high but strength is decreased. Hence, in order to make the bipolar plate in the form of a thin film to reduce the weight thereof, low strength and poor thermal conductivity in a vertical direction may result, thus lowering the energy production efficiency using waste heat, which is undesirable. On the other hand, in the case where a bipolar plate is manufactured using graphite having a small size, the graphite is easily stripped and is thus efficiently dispersed in a polymer matrix, thereby enhancing strength, but the graphite particles are not connected to each other, undesirably increasing electrical resistivity.